The present invention relates to moisture-curable resin compositions, to methods for their manufacture and to processes for bonding substrates, e.g., metal sheets and panels, employing moisture-curable resin compositions.
Prior art hot melt adhesives and sealants are typically thermoplastic polymers containing tackifiers, fillers and other additives. These hot melt compositions are solid at room temperature but flow at elevated temperatures. Because the compositions are made up primarily of an uncrosslinked thermoplastic component, they are often of low modulus and are susceptible to creep and cold flow under static loads making them unsuitable for many applications where strength of the adhesive joint is critical such as that required for bonding together heavy metal sheets in the construction of truck trailers. The weight on the joints, combined with vibration and environmental heat and moisture resulting from exposure to sun in hot and humid climates, can cause adhesive joints made with known hot melt adhesives to distort and eventually fail.
Moisture-curable hot melt adhesive compositions are known in the art which partially address the foregoing deficiencies. These compositions often contain a thermoplastic component such as a chlorinated paraffin or plasticizer, styrene block copolymer, butyl rubber or poly-α-olefin, and a silylated polyurethane based upon polyols or polyamines containing polybutadiene, polyester, acrylic, polycarbonate or polythioether backbones. Moisture-curable hot melt adhesive compositions contain a continuous phase of the thermoplastic component in which the silylated urethane is dispersed and crosslinked during the application process. The thermoplastic continuous phase, however, remains susceptible to creep and cold flow when placed under static loads. Where the silylated polyurethane component of a moisture-curable holt melt resin composition is incompatible with its thermoplastic component, little if any crosslinking will occur between the two thus giving rise to phase separation of the cured material. Single component and moisture-curable hot melt compositions may therefore exhibit insufficient strength and modulus due to the thermoplastic polymer debonding from the silylated polyurethane under static loads. The silylated polyurethane phase may also be too elastomeric and too low in modulus to provide the desired levels of bond strength.
Hot melt adhesive compositions often exhibit poor “open time.” Open time is used herein to refer to the “working time” of an adhesive which begins at the point when an adhesive is applied to a first substrate until it is bonded to a second substrate. It is critical that the adhesive present on the first substrate be joined to the second substrate during this time. If the open time is exceeded before the first substrate is joined to the second substrate, the adhesive will lose its ability to bond the second substrate. Longer adhesive open times permit greater flexibility in manufacturing bonded articles and the use of automated apparatus such as robotic assembly machinery.
Silylated acrylic polymers are known in the art, e.g., as disclosed in U.S. Pat. No. 4,491,650, where they are used as primers. However, these polymers are unsuitable for use as adhesives due to their brittleness.
Published U.S. Patent Application No. 2006/0173121 describes a moisture-curable composition containing silylated polyoxyalkylene polymer, copolymers of methacryloxysilane, acrylic acid alkyl ester monomers and an ionic surfactant. However, the glass transition temperatures for these copolymers must be lower than 0° C. for flexibility. These moisture-curable compositions therefore have low modulus and as such may be unsuitable for applications in which high bond strength is required.
There remains a need in the industry for moisture-curable resin compositions for use, inter alia, in hot-melt adhesives and sealants which are solid at room temperature, flowable at elevated temperature and cure to high modulus products that do not exhibit significant cold flow or creep under static loads. The present invention, as described more fully herein, provides such compositions.